Single-Electron Response Using a GEM-MIGAS Electron Multiplier

A gas electron multiplier with a micro-induction gap amplifying structure (GEM-MIGAS) is formed when a conventional GEM is operated with a short induction gap, typically set at 50 mum. Experimental studies were carried out to investigate the single-electron response of a GEM-MIGAS, using a He/iso-C₄H₁₀ (85/15%) gas mixture operated in a flow mode at atmospheric pressure. The additional charge multiplication in the induction gap results in a gain increase up to one order of magnitude when compared to the GEM mode operation alone. A series of measurements were undertaken to examine the pulse height distributions induced by single-electrons under a wide range of bias voltages applied across the GEM holes, 100 to 550 V, and for a wide range of electric fields in the induction gap, 0.6 to 100 kV/cm. It was possible to sustain effective charge gains in excess of 3times10⁵ B and multiplication relative variances around 0.4 over a large range of GEM voltages, enabling us to demonstrate single-electron detection efficiencies above 98%.